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    <title>Document</title>
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    let num1 = 123,num2 = 'hello';
    console.log(num1);
    console.log(num2);
    console.log(typeof(num1));  //打印数据类型
    console.log(typeof(num2));
    console.log('-------');
    let flag = true;
    console.log(flag);
    console.log(typeof(flag));
    flag = false;
    console.log(flag);
    console.log(typeof(flag));
    console.log('--------');
    let str1 = '子曰："学而时习之"';
    console.log(str1);
    console.log(typeof(str1));
    //子曰：'学而时习之'
    //he said:'day day up';
    //在字符型中，单引号中用单引号，或双引号中双引号，需要用到转义符'\'.
    let str2 = 'he said:\'day day study\'';
    console.log(str2);
    console.log(typeof(str2));
    str2 = "子曰：\"学而时习之\""
    console.log(str2);
    console.log('=-------');
    let Null =null;
    console.log(Null);
    console.log(typeof(Null));
    console.log('-------');
    let unNum;
    console.log(unNum);
    console.log(typeof(unNum));
    let str3 =
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